When attaching a chip, which is also referred to as integrated circuit, to a carrier utilizing, for instance, flip-chip technology, then contacts of the chip may be connected to connection pads of the carrier. One example of such a scenario is the assembling of an RFID transponder being comprised of an antenna and a chip connected to the antenna.
FIG. 1 shows a topview and FIG. 2 shows a sideview of an example of such a chip 1. The chip 1 comprises a main body 2 which is basically a substrate made, for instance, from silicon enclosing a circuitry as it is commonly known in the art.
The chip 1 comprises a first chip contact pad 3 and a second chip contact pad 4 as contacts for the circuitry. For contacting the chip 1 to contacts of a carrier, the chip 1 comprises a first bump 5 applied on the first chip contact pad 3 and a second bump 6 applied on the second chip contact pad 4 for this example.
An impedance, Z, between the first and second chip contact pads 3, 4 is the input impedance of the circuitry of the chip 1. Its equivalent circuit is depicted in FIG. 3.
The input impedance, Z, can be modeled as a resistance, R, connected in parallel to a capacitor, C. Additionally, one of the two bumps 5, 6 may be connected to the main body 2 of the chip 1 via an electric connection having a relatively small resistance and/or a relative large capacitance. Neglecting this capacitance, which is particularly justifiable if the input signal for the chip 1 is a high frequency signal as it is normal for an FRID transponder, or neglecting this resistance, then this connection can be modeled as a short circuit 7 between the relevant bump 5, 6 and the main body 2. For the example shown, the second bump 6 is connected to the main body 2.
In order to contact electrically the chip 1 to contact pads of the carrier, the first and second bumps 5, 6 may be bonded to these contact pads.
FIG. 4 shows a topview and FIG. 5 shows a sideview of the chip 1 attached to a carrier, which is a plastic foil 8 for this example. The foil 8 comprises a first contact pad 9 and a second contact pad 10. The first bump 5 of the chip 1 contacts the first contact pad 9 and the second bump 6 of the chip 1 contacts the second contact pad 10 so that the chip 1 and the foil 8 are spaced apart. If the foil 8 carries an antenna 11, as it is the case for the example shown, then the contact pads 9, 10 may be contact terminals of the antenna 11.
The main body 2 of the chip 1 overlaps partly the connection pads 9, 10 of the foil 8. The overlapping areas are denoted by the reference signs A1 and A2, wherein the overlapping area A1 relates to the overlapping between the first contact pad 9 and the main body 2 and the overlapping area A2 relates to the overlapping between the second contact pad 10 and the main body 2 of the chip 1.
Each of the overlapping areas A1, A2 form stray capacitors with the main body 2 of the chip 1. Particularly, the overlapping area A1 results in a first stray capacitor, C1, between the first contact pad 9 and the main body 2 and the overlapping area A2 results in a second stray capacitor, C2, between the second contact pad 10 and the main body 2.
The resulting capacitances can be calculated according to the following equations:
            C      1        =          ɛ      ⁢                        A          ⁢                                          ⁢          1                d                        C      2        =          ɛ      ⁢                        A          ⁢                                          ⁢          2                d            
wherein d is the distance between the contact pads 9, 10 of the foil 8 and a surface 12 of the main body 2 of the chip 1 facing towards the foil 8 and ε is the permittivity.
A resulting equivalent circuit can be modeled as shown in FIG. 6. Since the second chip contact pad 4 is basically connected to the main body 2 of the chip 1 by the short circuit 7, the second stray capacitor, C2, can be neglected. The first stray capacitor, C1, however, has the effect that the resulting input capacitance, Cres, of the circuitry of the chip 1 mounted on the foil 8 becomes:Cres=C+C1 
Particularly, when producing the foil 8 including the chip 1 attached thereon in relative high quantities, then the overlapping areas A1 of different assembled foil 8 chip 1 combinations are likely to differ, resulting in different resulting input capacitances, Cres, of the assembled combinations. Particularly, if the resulting input capacitance, Cres, affects the performance of the combination, as it may be the case for an RFID transponder, different resulting input capacitances, Cres, may affect negatively the performance of individual assembled foil 8 chip 1 combinations.